Background Prevalence of overweight and obesity is increasing both in Denmark and in most other countries, and as conventional treatment of obesity with lifestyle changes (hypocaloric diet, physical activity and behavior modification) rarely leads to large and sustained weight loss, it is not surprising that bariatric surgery has taken over more and more of the treatment of the most severe cases of obesity. Weight losses are greater than with other treatments, and importantly the relatively large weight loss is permanent. The currently most commonly used surgical method is the gastric bypass (RYGB), which has proven to be a highly effective method of achieving a great and permanent weight loss with an approximately 30-40% reduction in the weight, which is equivalent to 40-60% reduction of the overweight. Moreover RYGB has shown to provide complete remission of prior type 2 diabetes in up to 80% of cases at short observation time (up to 2 years after surgery) and about 50% remission at longer time follow-up (> 5 years).